Explained by D.P. Cox, Steve Ludington, B.R. Berger, M.G. Sherlock, and D.A. Singer, (USGS); and J.V. Tingley (Nevada Bureau of Mines and Geology)
On the choice of deposit models
Quartz-adularia veins have been subdivided into three subtypes, Comstock, Sado, and Creede, based on their metal grades and the presumed character of the basement underlying the volcanic sequence in which they are found (Mosier, Singer, and others 1986). The Comstock subtype, rich in silver and low in base metals, is generally found in volcanic rocks overlying low-grade metasedimentary basement rocks, and is, by far, the most abundant subtype in Nevada. The Creede type, rich in base metals, is not found in Nevada, and the Sado type, with low silver to gold ratio, makes up a very small proportion of the known deposits. Therefore the Comstock grade-tonnage model was applied to undiscovered quartz-adularia districts.
On the delineation of permissive tracts
The known Comstock-type deposits are distributed in a crescent-shaped area, concave to the east, that corresponds poorly with the overall distribution of Tertiary volcanic rocks (Silberman and others, 1976; Stewart and others, 1977; Seedorff, 1991; Cox and others, 1991; Ludington and others, in press). This distribution of volcanic-hosted epithermal deposits cannot be explained by the absence of volcanic rocks inward from the crescent. On the contrary, eastern Nevada contains extensive outcrops of older interior andesite-rhyolite assemblage rocks (older than 27 Ma) in which epithermal vein deposits are virtually unknown.
In addition to active volcanism, faulting and fracture permeability are important in controlling the distribution of epithermal deposits. The crescent-shaped area described above corresponds closely to those areas which were undergoing faulting in an extensional tectonic regime during active volcanism. The synvolcanic deformation is important because it provides fracture permeability at the same time that hydrothermal systems related to volcanism are active and circulating, thus facilitating the formation of veins and stockworks. Where Miocene volcanic rocks are relatively unfaulted, for example in the Sierra Nevada of California and in the Cascade Range of Oregon and Washington, Comstock deposits are rare or absent.
The Walker Lane area contains well-developed normal faults, and is probably the best studied region of epithermal mineralization in Nevada (Stewart, 1988). Northwest-striking high-angle faults that predominate in this area have been shown by John and others (1989) to be at least as old as the earliest volcanic activity (22 Ma) in the Paradise Range suggesting that faulting and volcanism were synchronous throughout the period of andesite volcanism. This region is shown by Blakely (1988) to be characterized by a northwest-trending grain in the pattern of magnetic anomalies that can be recognized about 50 km to the northeast of traditional boundaries of the Walker Lane that are based on topography and structure. This expanded area of characteristic magnetic fabric encompasses all of the volcanic-hosted epithermal districts in southwestern Nevada. The northeastern boundary of this magnetic anomaly pattern coincides with a line separating calderas younger and older than 27 Ma. (Best and others, 1989); the eastern boundary is the magnetic quiet zone (Blakely, 1988). Walker Lane deformation began locally at 27 Ma, and continued during succeeding volcanic episodes until the beginning of Basin and Range deformation at about 11 Ma, thus controlling the distribution of epithermal precious-metal deposits in this part of Nevada. A strong negative correlation exists between the magnetic quiet zone and the distribution of volcanic-hosted epithermal deposits. This is especially clear in the southern arm of the crescent where a gap exists between the deposits in the Walker Lane and the Atlanta and Stateline districts to the east in Lincoln County.
The permissive tract for quartz-adularia districts is based on the distribution of volcanic rocks, of epithermal mineral deposits, prospects, and occurrences, on the distribution of synvolcanic faults, and on the magnetic anomaly patterns described above. This tract covers about 55 percent of Nevada; about 47 percent of the tract is covered by superficial deposits younger than the mineralized rocks. Because some epithermal deposits occur in sedimentary rocks close to volcanic centers (Willard, Atlanta, Florida Canyon), sedimentary rocks within and between the volcanic rock areas are included in the tract.
Important examples of this type of deposit
The Comstock and Tonopah districts, the largest in Nevada, are associated with volcanic rocks of the western andesite assemblage. The Jarbidge and National districts are related to the bimodal assemblage, and Tuscarora, to the andesite-rhyolite assemblage
On the numerical estimates made
Our estimate of the number of undiscovered districts of the quartz-adularia type was based on the following considerations:
(1) A district is defined by the grade-tonnage model for Comstock epithermal veins (Mosier and others, 1986b). Roughly 30 such districts are known in Nevada.
(2) An additional 8 districts are known that are too small to fit the definition. Some of these may be incompletely explored and present opportunities for the discovery of new districts.
(3) Quartz-adularia vein systems will probably be found in the vicinity of some hot-spring gold deposits as exploration near these deposits proceeds. Where the hot-spring deposit is isolated from other known epithermal districts, these systems are considered to be evidence for undiscovered quartz-adularia districts.
(4) Quartz-adularia vein deposits can be detected by prospecting methods that have been employed in Nevada since the 1850s. Thus exploration for them in exposed permissive areas can be considered to be well advanced.
(5) Known deposits and prospects are mainly in areas of exposed permissive rock. Only one deposit, Sleeper, has been found beneath alluvium. Thus, the 47 percent of the permissive area under cover is likely to contain many undiscovered deposits.
For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 14, 18, 24, 26, and 29 districts consistent with the Comstock grade and tonnage model (Mosier and others, 1986).
Best, M.G., Christiansen, E.H., Deino, A.L., Grommé, C.S., McKee, E.H., and Noble, D.C. 1989, Excursion 3A: Eocene through Miocene volcanism in the Great Basin of the western United States: New Mexico Bureau of Mines and Mineral Resources Memoir 47, p. 91-133.
Blakely, R. J., 1988, Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada: Journal of Geophysical Research, v. 93, p. 11, 817-11, 832.
Cox, D.P., Ludington, Steve, Sherlock, M.G., Singer, D.A., Berger, B.R., and Tingley, J.V., 1991, Mineralization patterns in time and space in the Great basin of Nevada, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and ore deposits of the Great Basin—Symposium proceedings: Reno, Geological Society of Nevada, v. 2, April 1990, p. 193-198.
John, D.A., Thomason, R.E., and McKee, E.H., 1989, Geology and geochronology of the Paradise Peak Mine and the relationship of pre-Basin and Range extension to early Miocene precious metal mineralization in west-central Nevada: Economic Geology, v. 84, no. 3, p. 631-649.
Ludington, Steve, Cox, D.P., Sherlock, M.G., Singer, D.A., Berger, B.R., and Tingley, Joe, in press, Spatial and temporal analysis of precious-metal deposits for a mineral resource assessment of Nevada: Ottawa, Canada, Transactions IAGOD/IUGS Symposium, 15 p.
Mosier, D.L., Singer, D.A., Sato, T., and Page, N.J, 1986, Relationship of grade, tonnage, and basement lithology in volcanic-hosted epithermal precious- and base-metal quartz-adularia-type districts: Mining Geology, v. 36, no. 4, p. 245-264.
Mosier, D.L., Sato, Takeo, and Singer, D.A., 1986, Grade-tonnage model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 151-153.
Seedorff, Eric, 1991, Magmatism, extension, and ore deposition of Eocene to Holocene age in the Great Basin—Mutual effects and preliminary proposed genetic relationships, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and ore deposits of the Great Basin—Symposium proceedings: Reno, Geological Society of Nevada, v. 1, April 1990, p. 133-178.
Silberman, M.L., Stewart, J.H., and McKee, E.H., 1976, Igneous activity, tectonics and precious metal mineralization in the Great Basin during Cenozoic time: Society of Mining Engineers, AIME Transactions, v. 260, p. 253-263.
Stewart, J.H., 1988, Tectonics of the Walker lane belt, western Great Basin—Mesozoic and Cenozoic deformation in a shear zone, in Ernst, W.G., ed., Metamorphism and crustal evolution of the western United States (Rubey Volume VII): Englewood Cliffs, New Jersey, Prentice Hall, p. 683-713.
Stewart, J.H., Moore, W.J., and Zietz, Isidore, 1977, East-west patterns of Cenozoic igneous rocks, aeromagnetic anomalies, and mineral deposits, Nevada and Utah: Geological Society of America Bulletin, v. 88, p. 67-77.